How to protect the inputs against Common Mode (CM) voltages exceeding the supplies, when power is off or on?
I cannot use a series resistor of more than 10 ohms because of noise. That resistance does not provide much protection, so I need to depend on other types of protection.
I can accept a system in which 100mA of current flows before the safe voltages are exceeded. Typically, the user will have a finite source impedance, so 100mA will give useful protection.
The input is protected against differential voltages by the "front to back" low leakage diodes D2 shown in the schematic.
When the power is on, the diode combination (D1-D4) breaks down at about +/- 14V.
However, when the power is off, the Zener diodes will not help.
The only solution I could think off was to add a Normally Closed relay, connecting the inputs to ground when the power is off.
The remaining weakness is that if the user shuts off power during a large input signal, the input is not protected until the relay closes.
About 5-8pf has been added to each input due to the relays and diodes.
Are there any other methods which will not add a lot of noise or capacitance?
Would it be better to put the relay in series with the input signal?
Any other thoughts or ideas on this?